Automatic fidelity control circuit



June 22, 193 7.

R. A. BRADEN AUTOMATIC FIDELITY CONTROL CIRCUIT Filed July 9, 1935 5 Sheets-Sheet l 2 w114/L. ATTORNEY R. A. BRADEN AUTOMATIC FIDELITY CONTROL CIRCUIIl Filed July 9,A 1955 June z2, 1937.

lNvENToR i E/VEAMQE/V BY )VQ ATTORNEY June 22, 1937. R A BRADEN 2,084,379

AUTOMATIC FIDELITY CONTROL CIRCUIT Filed July 9, 1955 5 Sheets-Sheet 5 #iw-WWII- VIIII "Q I MllL hmmm o FWF 00.00A... OOCOCOAOO ATTORN EY June22, 1937. R. A. BRADEN `2,084,379

AUTOMATIC FIDELITY CONTROL CIRCUIT Filed July 9, 1935 5 sheets-sheet 4 Hylllblw- ATTORNEY Patented June 22, 1937 l UNITED stares AUTOMATIC FIDELITY CONTROL CIRCUIT Rene A.,Braden, Collingswood, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application July 9, 1935, Serial No. 30,443

6 Claims.

My present invention relates to circuits for regulating in an automatic manner the fidelity of radio receivers, and more particularly to improved systems for automatically varying the selectivity of a radio receiver in response to received signal amplitude variations.

In my co-pending application Serial No. 10,981, filed March 14, 1935, patented September 8, 1936 as U. S. P. 2,053,762, there are disclosed, and 10 claimed, improved arrangements for automatically decreasing the selectivity and improving the fidelity of a radio receiver when receiving strong signals. In the` said application the arrangements are each shown as comprising parallel sharp and broad signal amplifiers. Each of the ampliers feeds a .demodulator and amplifier gain control device; the gain of each amplifier is reduced. with increasing signal strength, but the gain of the sharp amplifier is reduced at a much faster rate in order to have the broad amplifier predominate in signal transmission at strong signal amplitudes.

Now, in the present application, there are disclosed additional improved circuits for regulat- 5 ing the signal outputs of sharp and broad amplifiers of a receiver system ina manner such that the signal output of the sharp amplifieris very little at strong signal reception, the circuits be-v ing characterized by the utilization in each circuit,l

of a multiple function tube which operates to detect the output of' each regulated amplifier and simultaneously to perform the amplifier gain regulation. Y

Accordingly, it may be stated that it is one of the main objects of my present invention to provide at least two parallel amplifiers in a radio receiver, one of which amplifiers has a sharp selectivity characteristic and the other one having a broad selectivity characteristic, the receiver additionally including control circuits for differentially controlling the transmission, efficiency of said amplifiers in suchamanner that as the signal amplitude input to the receiver increases, the gainv o both amplifiers decreases, the gain of the sharp amplifier decreasing more rapidly than that of the broad amplifier, the receiver further comprising an electron discharge device which simultaneously performs the functions of demodulating the output of each of the amplifiers,v

to provide in. a radio `receiver at least oneI ampli?.

fier having a broad selectivity characteristic and 55 at least one other amplifier having a selectivity and providing the control bias for regulating the;

(Cl. Z50-20) characteristic which is sharp, the amplifiers-having electrically associated therewith a multiplev duty electron discharge tube whichsimultaneous-l ly functions to demodulate the output energies of the amplifiers, and additionally regulates the gain of each of the amplifiers in such a manner that with received signal amplitude increase the gain of the sharp amplifier is caused to decrease more rapidly than that of the broad amplifier with the result that the over-allselectivity characteristic of the receiveris broad when receiving strong signals and sharpwhen receivingfweak signals.

Another important object of the invention is to provide in a radio receiver of the superheterodyne type, apair ofl intermediate frequency amplifiers of sharp and broad*selectivityresin3@- tively, the amplifiers having associated ywith them gain control diode rectiers which function to regulate the gain ofeach ofthe amplifiersin such a manner that as the signal ,carrier ampli-.- tude increases, the gain of the sharp. amplifier decreasesv more rapidly than that of= the broad amplier, the diode rectiers additionally functioning to demodulate the output currents of: the amplifiers, and the system including additional means for-causing the gain of the broad-amplier to decrease when receiving signals below a predetermined signal'amplitude level Wherebythe-` selectivity of the receiver is improved when receiving Weaksignals by cutting-the broad-amplifier completely out of service.

Still otherfobjects Vof the invention are toim prove generally automatic iidelitycontrol circuits for radio receivers, and to provide such control; circuits which are not onlyvreliable in operation, but readily constructed and assembledin radio receivers. Y

The novel features rwhich Ibelieve to be characteristic of my invention are setforth inP particularity in the appended claims;A the invention itself, however, as to both its organizationA and methodof operation will best be understoodby. reference to the following description talggen,inh connectionpwith the drawings in whichl have` indicated diagrammaticvally several circuit organizations whereby my invention Vmay be carried intofeffect. I y V In the draWings;- Fig. 1 is a circuit diagram of a superheterodyne receiving system embodying the invention. Fig. 2 graphically shows the signal inputfsignal. output characteristicsv of. the. Asystem shownin. Fig. 1, l Y

'.1 5 shows an .2 c c Y Y Y Fig. 371sV afcircuit diagram of amodified form of Vthe invention, Y

Fig. 4 shows still another embodiment of the invention,

tion, o e y vFig. 6 shows a modification of the invention `wherein the fidelity control'is manually operated;`

Referring now tothe accompanying drawings,

r l() wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. l, in `purely conventionalform,

a radio broadcast `receiving arrangement em- Ybodying one jembodimentof the invention. The

, Vl5 receiving arrangement comprises a source of sig- Vnals of intermediate frequency, it'being'pointed out that the receiving arrangement is ofthe superheterodyne type., It will be understood that, although the following circuits are illustrated as 200i the superheterodyn'e type, the invention isv equally applicable' to receivers of the tuned radio Y frequencytype or anyother well known type of signal receiving systenn Y The source of intermediate frequency signals Y Vshownriri Fig. Y1 "mayincludaas is well known to those skilled in the radio art, a source of signal energy such as a signal collectingV meansj a tunable radio frequencyampliiier; 'and' the'output of the latter being coupled toa frequency changer network of any desired type. For example, the frequency changer device may be of the combined local oscillator-first detector type or it may be of the type `wherein a separate first detector tube isV utilized and upon which is impressed local oscillal tion energy fromany desired local oscillator tube arrangement. Regardless of the type oi receivingv arrangement Vemployed prior to the intermeaV j diate frequency amplifier network, the interme-V diate frequency energyris impressed upon the in- 40 Vput-terminals ofzthe pair ofY parallel amplifiers Yshownin Figi.

- 'eioutput'of `@anni amplifier' networkis ,impressed upon a, multipiefunctionftube l.. This j 1 tube may'be of the-55,ror 85, type, and usually comprisesapair Lofgindependent diode sections as Well as an amplifier section. vThe specific construction Voftube l. is.. not. necessary to the dis# closure'of the present application because this type of tube is at Vpresent rwellknown to those Y skilled; infthe art.v It is sufiicientj to pointoutv that itcomprises avcathode providing anelectron streamto diode anode 2 and diode anode 3; a, grid and plate are usually provided tofurnish an indef Y The electron Y stream Vto the gridlandplate is independent of the elec- Y pendent amplifier section. Y

o tronstream to the diode anodes, itbeing understood, that-the electronV streams vflowing toV the diode anodes from the cathode are also independent. a Y, V' 'Ir'he diode anode 2 is connected to the cathode "of, tube l throughra path which includes in series the tuned input A' cathode side vof resistorfR sistor Ris shuntedlby ra radioffrequency by-pass lcondenserjr.' lThe diode anode 3 is connected to Y the cathode of tube I ythrough a series path which includesthe' tunedinputcircuit 6 and theV load resistor R; The tuned input kcircuit 4 includes YVthe secondary winding of thecoupling'trans;

formergMz, and the primary winding of this transformer is Valso'of the tunedftype. The ampliiier network VVfeeding the Ytilinedecirc'ut V4 is Y`designated as beingsliarply tuned; that is to say,

. it possesses additionalV form of'` the invencircuit Y4f and resistor R. TheV is grounded, and reV Y l, aselectivityfcharacteristic which isl 1 7 5 relatlvelysharpwitn respecta. the ga Y sharp amplifier is shown as comprising a pair of cascaded amplifier stages. Y The cascaded ampli j fier tubes are shown coupledbya transformer M1 Y which has itsprimary'and secondary winding cir- Vcuitseach tuned.V It will be understoodthat the tuned winding circuits of each :of thefcouplingf transformersrMl and Mrzjare resonatedto .the op-Y eratingY intermediate frequency. 'Y

Similarly in thercase ofthe broadly tuned amplifier, the transformer M3 couples theicasf ceded amplifier tubes thereof, andthe trans-` former M4 includes as its secondary circuit the tuned input-'circuit 6. The tuned'eircuits of these latter twotransformers M3 and M4 are each resonated to the operating intermediate frequency. VIt will now be seen that the signal output'curf rent of each of the parallel amplifiersis demodulated bythe diode rectifier circuits operativeily"v associated with tube l.A Thedi'ode rectifier circuit which includes the diode anode) and thecathodejl of tube I demodulatesitheoutput current Vof the` sharply tuned LF. amplifier, andthe other diodeV l rectiiier circuit demodulates the output currentof Y the broad amplifier. Y Y The audio frequency component of therecti fied signal currents is taken off from a, desiredl. o point on resistor R by theY adjustable leadl, and

the` audio V'frequency energy is transmitted toa succeeding Vvaudio 'frequency transmissionv network which may comprise one, or more, amplifier stages followed by any desired type of reproducen It will be noted that .adjustment ofthe vrlead 1 will regulate the volume Voi y reproducedA energy, and therefore, this adjustablelead may be consideredI as a manual volumer control device., It Vwillnowv be observed'that between the source vof intermediate frequency signals'and the input circuits ilk and 6 of the demodulator network, there is dis-H posed an amplifier system which includes one` channel sharplyi-,tunedtothe operating,intermeY-f--V K diate frequency;` as well vasranothef channel4V broadly tunedl to the same frequency? t i In `order to secure the different selectivity characteristics for the parallel I F. amplifier neteV l works, any'device well knownto those'skilledV inv Ytheart maybeutilized. l Onemethod offsecuring' Y such-'selectivity'characteristics, and it is to be ,L

clearly understood Ythat it is ugiven merelyby way of illustration, and not tolimit the invention, is to adjust the'coupling devicesMi andMz tor have `their coupling magnitudes less than critical so that'.

the resonance curve characteristic ofthe circuits;

associated with thesetransformers is relatively sharp. In the case oftransformers M3 an'd .M4 Y

they are designed so that their coupled effects areV greater thancritical coupling with the result that Y .thecircuits associated therewithvare given a rela- Ytively broad resonance curve characteristic.V

Y Preferably, the couplings M3 and Mi'are adjusted soV that the resonance curve of the broadly tuned, j amplifier system has a substantially flat top withV straight steep sides. Theresonance curve charaeteri'stic of the'sharplytuned I. Ffamplifierpref" er'ablylhas thefshape of an inverted V. Other `devices for securingvthesecharacteristicsifor the' YV-parano amplifiers wiu'readny Vsuggest {theiseli/esk fito those skilled inthe art, the essential feature of.l the` present invention requiring that the res'o nance Vcurve characteristics of the parallely amplf fiersbe those described heretofore. 1

` itis desiredto operate the parallel amplifiers in such a manner that as ,thegsignal input increasesY the'gaino'f both amplifiers'cdecreases,1but the gainV Y' than that of the broad amplifier. The result of this is that with weak signals the gain of the sharp amplifier is much greater than that of the broad one and the receiver output acts mostly through the sharp circuit, while with strong signals the gain of the sharp circuit is less than that of the broad circuit and the receiver output acts through the broad circuit only. This mode of operation is secured by connecting the tuned signal grid circuit of at least one of the cascaded amplifier tubes of the sharp amplifier to a point of negative potential on the load resistor R.

This connection is made through lead 8 to point 9 of the load resistor R. The lead il includes the usual network, comprising resistor II) and grounded condenser I I, for filtering out any pulsating components from the direct current conrol bias fed through lead 8 to the grid circuits of the controlled amplifiers. The signal grid circuits of the amplifier tubes of the broad amplifier are connected to a point on resistor R which is positive with respect to point 9. This is illustrated by the lead 8 connected between the signal grid circuit of the second of the cascaded amplifier tubes of the broad amplier and point I2 on resistor R. The lead 3 also includes the pulsating component filter network ity-l i and it will be observed that point I2 on resistor R is at a positive potential with respect to point B.

Those skilled in the art will recognize that the leads 8 and 8 can be connected to the various signal grid circuits of the amplifier tubes of each of the broad and sharp amplifiers. The circuit details of the cascaded tubes of the broad and sharp ampliiers are not shown, but it is pointed out that the cathodes of all tubes controlled by the AVC bias must -be connected to ground through a conductive circuit, in order to complete the circuit for impressing AVC bias on the grids. It will be seen that as the signal amplitude increases the control bias applied to the signal grids of the sharp amplifier system ismore negative than that applied to the tubes of the broad amplier. This means that the gain of the sharp ampliiier will decrease more rapidly than that of the broad amplifier. The broad amplifier network has a much lower maximum value of gain than the sharp amplifier, with no signal impressed on the amplier. As a consequence, with weak signals the gain in the broad amplier network is 4 insuicient to produce an appreciable amount of output for utilization in the audio transmission network, and the sharp amplifier circuit substantially feeds the signal energy to the demodulation network. However, when strong signals are received, the sharp amplier network is rendered inoperative since it is biased more'than the broad amplifier network, and as a result the signal energy is fed to the demodulator network through the broad amplifier channel. At intermediate signal levels, energy is supplied to the audio circuits through both amplifiers, in proportions depending cn the signal strength.

In Fig. 2 there are shown ideal signal inputsignal output characteristics ofthe sharp and broad amplifiers illustrated in the system of Fig. l. This figure shows that as the signal input increases, the input being measured in microvolts, the output of the sharp amplier at first remains practically constant andvthen decreases. The output of the broad amplier starts at a Very small Value and increases as rapidly as the output of the sharp amplierfdeoreases with the result that the total output remains substantially constant. The signal input-amplifier gain characteristics corresponding to the characteristics shown in Fig. 2, are not shown in the present application, but are shown in the aforesaid. co'lpending application. Briefly, such characteristics show that the gain of the broad amplifier networkis substantially lower than that of the sharp amplifier at low inputs, and gradually decreases becoming asymptotic to the curve of total gain for large signal inputs. The curve for the sharp amplifier coincides with the curve for total gain of the system at small input values and fallsV far below it at high input values.

In the aforesaid application, there is also disclosed the resonance curve characteristics of the sharp and broad amplifiers when arranged in parallel and regulated as shown in Fig. l of the present application. It is not believed necessary to reproduce such characteristics in the present application, and it is sufcient to point out that the ideal operation of the system shown in Fig. l is that which results in a resonance curve char'- acteristic for strong signals which possesses a substantially flat top wide'enough to respond to thc whole frequency spectrum of the modulated signal carrier, and straight, steep sides. For weak signals the ideal resonance curve possesses the shape of an inverted V. The complete resonance curve of the whole system of amplifiers is made to approach either of the two extreme ideal conditions, or to have an intermediate form, through the automatic regulation of the gains of the two parallel ampliers in the manner described heretofore. The ideal form of resonance curve may be approached as closely as desired by using a sufliciently large number of tuned and controlled amplifier stages.

In Fig. 3 there'is shown a modied form of the invention wherein the characteristics shown in Fig. 2 are secured, and in addition thereto, there is also secured a delay action on the broad amplifier gain regulation.v That is to say, the circuit arrangement in Fig. VV3 includes a device for decreasing the gain of the broad amplier below'a predetermined signal input ampli tude. Such a condition is preferable because it improves the selectivity slightly; -when receiving weak signals, by cutting the broad amplier completely out of service. In the circuit arrangement of Fig. 3 the sharp amplifier is shown as comprising the amplier tubes 2l), 2|, 22 and 23 arranged in cascade. The broad amplifier com prises the amplier tubes 2K9', 2l', 22' and 23` in cascade. Y

The signal input grid of tube 2li is connected to the high alternating potential side of the tuned input circuit 24 of amplifier 20. As previously explained the coupling circuits between the cascaded tubes in each of the parallel amplifier channels are arranged so as to impart the particular desired selectivity characteristic to the amplifier` system. The tube I is provided with the two diode rectifier circuits shown inrconnection with Fig. l. Therungrounded side of load resistor R is connected by lead 25 to the tuned input circuits of amplifiers 20 and 2l, and this lead is the gaincontrol connection to these amplier tubes.Y It will be observed that the gain control connection 25 operates on the signal grid of the rst of the broadamplifier tubes Zllfrby virtue of the 'connection between the grids of tubes 20 and 2li. Y'

VThe amplifier tube 2l of the sharp amplifier system includes in its grounded cathode lead resistor Ri, and the cathode side of the resistor is connected by4 leadV 26 tothe cathode oftube 2|'. Y

Theauxiliary control tube A2'! is provided for a purpose to be described in detail at a later point,

and this tube has its anode grounded; its cathode.s' connected tothe ung'rounded side of re sistor R through a pathY which includes resistor Rg and lead 2li.V The control grid of tube vZ'l is connected to the negative side of resistor Rg, and

gain control connections are provided fromthevv opposite terminals of resistor R2 to the sharp and broad ampliiier systems.- f

VThe cathode side of Yresistor R2 is connected to the signal grid circuits of ampliiier tubesV 22 and 23 through lead 29. Leadconne'cts the signal grid circuits of tubes 22Y andr23 to the negative side of Yresistor R2.

It will be noted Vthatfeach of the gain control connections 25, 29

and 30 include resistor-condenser networks forV suppressing the pulsating current components of the gain control bias current. Y Y Y In the circuit of Fig. 3 the I; F.7'signals divide at inputcircuit 24 and areV impressed upontubes Y andY 20'.` TheseV .two tubes are provided` f j with' automatic gain control Vbias inthe usual across the 4diode 'rectiiie'rrload resistor R. The

tubes 2| and 2| of the sharp and broad networks V:

manner from the direct current Voltage drop respectively Yare provided with the commonr cath- Y ode resistor R1. VThe signal grid of tube 2|V is controlled as tobias by the gain control connection 25,'and the signal grid of tube 2| has a vfixed negative bias. Variation of the biasing voltage transmitted through connection 25 causes Va diierential variation of the keffective biases of tubes 2| and 2|', and `thus a differential variation Y of gain through these two tubes. That is,`when the voltage impressed on'the grid ofr2| by lead Y Y V25 issmall, the total eiirective bias ofY 2| is small,` l' Y*that of 2|' is large, Vand-2|l has the greater gain,

2|Vbeing practically shut olii When the voltage on lead 25 is large, biasof V2| is large, that of V2|- is small, and 2|?! has'the greaterV` gain, v2| `being Y practically shut offl AtV intermediatei.conditions both tubes amplify. To get the greatest v*possible jvariation of gain in the two tubes, itV may be nec. Y

v essay'to insert a1 bias batteryin theV grid circuit of 2|; at the point marked X'on` the diagram,

with itspositive .pole toward they grid *.The difr-v ferential V*bias variation l is produced by the common cathode resistor Ri.. As the bias on the Y grid of* 2|lincreases,'the.Voltage drop acrossjRi Y decreases because Yof the decreasint.;l space; ourrent through tube -2|. The voltagev drop across R1 produces aV negative bias voltage in the grid circuit of tube2lf, and this bias decreasesV as the bias on the Ygrid of 2| increases.

Y gam dhtml Connection 30' which is rivedefromY aVv O Yoltage dividing resistorY R3 Inf parallel with The tubes 22' andjza' arecontroied by tbe thisrvoltage dividing resistor ar'elconnected "the control tube 2`|Vin series with resistor'Rz. The

combination ofjRz in seriesjwithj tube z1,=the

latter being biased'byy the Voltagedrop'across R2, L tends to maintain a-:constant'current'through; Rail As the-voltage Vo'nflead 2,8Y increases, the ourc rent `increases slightly,y and this increases the grid bias and hence the voltage drop in the-plate Due to the ampliiicationV of the slightV change'in 'voltage across R2 is produced when the impressed voltage changes,

' most of the changeV Voccurringjfin the voltage ac rosslthe tube; When the impressed'voltage Vis that` across R2,Y but Vit `Yincreases rapidly` as the VoperatingY untilV the signal Vlected limiting value'.

small, thevoltage across 21 is` much smaller than-v impressed voltageV isAk increased, becomingj much larger than the voltage across Raj Y 'l The voltage fdrop'across control tube 2T is ap- Y plied ras an automaticA gain control bias to`V tubes 22 and '23 .through .the connection 29; As the received signalstrengtn increasesythe 'control Y Y bias applied through connection 29 increasesp faster than that applied through connection 30 peutV comes from the broad ampliiier. Withrweak tive, as explained above. It will, therefore,"be

seen that there is provideda circuit arrangei' ment wherein theV broad ampliiier is substantially inoperative for signals below a predeter-V Amined amplitude, and wherein the gain of both amplifiers is decreased as the signalstrength,in-Y creases, the sharp amplifier decreasing in kgain at arijaster rate than the broad ampliiier. i

Inthe circuit arrangement of Fig; 4, the ampliier section of the multiple `function tube is utilized. The negative terminal of diode rectiiier load resistor R is connected through thegain controlconnection 40 to the `tuned grid circuits Y of the amplifier tubes of the sharp amprliiiernet-V work.-' An intermediate point ongresistorfR is connection 4| tothe grid circuits 'off all of-the ainpliiers'Yof the broad amplifier network Vwith the exception rof' one ofthe amplifier tubes 42. The intermediatepoint 43 .on loadrresistor R is resistor RV to which lead 4|) is connected.

resistor-condenser network forsuppressing pulf satingV Vvoltage components. yI'he plate circuit oi the multiple function tube includes the f Y 30. connected through thegain` controlfregulationf Y 35 positive in potential with respect to thepoi'nt on- The` grid of tube is connected to the negative' 'Y Y`side Voi resistor R, and the connection includes a 2 positive potential source 43 and resistor 44, the 'Y resisto-r having one side thereof: groundedl Ai gaincontrol.connection 45 is provided between grounded side of Vresistor vM. Y

` 'In the circuit arrangement of Fig. 4,Y the voltage drop across resistor `44 is v`large whenthereis nofsignal beingfreceived, andsthis isV so yby virtue ofthe fact tbatthe gridbf tube is at its.

minimum bias whensubstantiallyV nosignals are received.VY As theY received; signal]amplitude,inz-777k 55' creases,- the voltage drop across-.resistorv'M'diminishes since the negative `bias amplitude increase. sistor V41| is applied e on the signalY Y grid ot tube lincreases withY received signal?. `The voltage drop across ref as a suppressing'bias to .oneg'v Y or more, tubes in the broad amplifier networkf@ 45 vthe tunedninputgrid circuit ofV theampher 42 fifi V of. the broad amplifier network and4 the un-1 This keeps thebroad amplifier`A network fromYV strength is above ase#V This limiting; 0r delay, action is made'bymn; i

necting Vthe'grid of the triode section of tube to a'predetermined pointv on therload resistorR; j v or by regulating' Ythe magnitude of resistor 44'; t or by applying Vanadditionaliixed bias 46. to the' tube` 42 to which the suppressingbias isapplied.

Y Otherwise, the *action of, the circuitshown; in V:1 Vis substantially lsimular tothat explained in" connection with 1, and 'it VVwill Y be recognized?Y 'y thatthe gain control Connectionsemployed inv this iigureY are substantially similar Yto those 1 shown in Fig. 1, with; the fmcepubnY tbatithe broad vamplifier operation delay circuit-is shown in Fig. 4. A

From another viewpoint'the functioning of th gain control connection 45 is somewhat similarin manner to the connection 26 shown in Fig. 3 between the cathodes of tubes 2|4 and 2|', The essential purpose Yof such a connection is to keep the broad amplifier out of service below a predetermined signal'input amplitude, this resulting in an improvement in selectivity when receiving weak signals.

An alternative arrangement which may be employed in place of the circuit shown in Fig. 4, is illustrated in Fig. 5. In this case the resistor 44 is included in the cathode lead of the triode sec-l received, there is a voltage developed across resistor 54, and substantially no voltage when the received signal amplitude is large. The voltageV Idrop across resistor 44 biases the cathode of tube 42. When receiving weak signals, the operation of the broad amplifier network is prevented by the bias connection operating through lead 45; but as the signal strength increases the bias on the cathode of tube 42 is removed, and the broad amplifier network is allowed to operate. Thus, it will be seen that there is provided an alternative method of delaying the operation of the broad amplifier network for signals below a predetermined amplitude.

In Fig. 6 there is shown an arrangement for regulating the selectivity characteristic of the receiving system by manual control. The sharp amplifier tubes each have their cathodes adjustably connected to a grounded resistor. Thus, the cathode of tube 5| of the sharp amplifier is 'connected by an adjustable connection 52 to the resistor 55 having one terminal thereof free and the other terminal grounded. In the same manner tube 5l has its cathode connected by an adjustable tap 52 to a resistor 55 having one terminal thereof grounded. The broad arnplifier includes in the cathode circuit of each of its tubes 53 and 53', an adjustable tap. Thus, tap 54 may be connected to any point along resistor 55, it being noted that the terminals of resistor 55 are free, whereas an intermediate point thereof is grounded. The same-construction applies to resistor 55 and tap 54' associated with tube 53'.

'Ihe taps 52 and 52 are mechanically coupled to operate conjointly, and the taps 54 and 54 are similarly mechanically coupled to operate simultaneously. Any desired type of Amechanical coupling Inay be utilized to simultaneously adjust the ganged pairs of taps of the sharp and broad amplifiers. As weak signals are received, the manual control Vinstrumentality is adjusted to move the taps 52 and 52 towards the grounded side of resistors 50 and 50'; simultaneously, the taps 54 and 5d are adjusted in a direction away from the grounded point on resistors 55 and 55.

Thus, the gain of the broad amplifier will be decreased whereas the gain of the sharp amplifier is increased. In the position shown in Fig. 6 the manual fidelity control knob has been adjusted for reception'xof afsignallof medium strengthg-and in thi'sposition' theA gain of the broadamplier network is -at a maximum, Whereas the 'sharp amplifier is operating at a value of gain consid#l erably. reduced from its maximum, but ofthe same order of magnitudeas that ofthe broad amplifier, so that each amplifier makes a sub-V stantial contributionto the total output, and the selectivity has an intermediate'valuebetweenthe adjustment the same effect that was gotten automatically in preceding circuits: i. e., ratio of output of sharp ampliner to output of broad amplifier decreasing as input signal amplitude is increased, Vthe broad amplifier being substantiallyA shut off when signal is very weak, and the sharp amplifier being substantially shut off when signal is very strong. 1t 1s assumed that the manual adjustment will be adjusted to give about the same signal output and amplitude regardless of the input amplitude, .though small variations within the usual range of variation of loudness of sound output to suit Idifferent tastes in repro'- duction of broadcast signals will not upset the predetermined relation between signal strength and selectivity and fidelity, since such variations are relatively small.

The arrangement of volume control rheostats shown in Fig. 6 is well adapted to the common form of rheostat in which the resistance material is circular in form, and the tap rotates on a concentric shaft. All the taps may beyarranged to be turned by a single shaft, as ndicated by the dash line in the diagram. Suitable relations between the gains of the several controlled amplifier tubes can be gotten by proper selection of maximum resistancewvalues of vthe different rheostats; proper grading ofv resistance change per unit angle of rotation of the control shaft; proper selection of the ground connection point o-n rheostats 55 and 55'; and passage of a steady current from thereceiver power supply (or a separate current source) through any or all of the rheostats in addition to the space current of the tube, or tubes.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited Yto the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a signal receiving system comprising a pair of parallel amplifier networks, one of the networks having a sharply selective characteristic and the other network having a broad selectivity characteristic, an electron discharge tube provided with a cathode and a pair of anodes, one of the anodes and said cathode being connected to a circuit which includes the output of said sharp amplifier and a load resistor, the cathode and remaining anode being connected to a circuit which includes the output V"of said broadzamplifier and said load resistor, an'

audio frequency transmission path 'connected to apoint on said load resistor, a gain regulation path connectedbetween at leastone of the tubes of said sharp amplifier anda point kof negative potentialonsaid load resistor, andfajsecond gain regulatiompath connected between `one ,of the tubes of said broad amplifier and ajpoint on said load resistor whichisata positivepotential with 7110i respect to said point o f negative potential. Y

U2.' In a signal receivingV systemrcomprising Va pair-fof vparallel amplifier networks, oneof they networks having a sharply selective character'-V istic and theothernetwork having ak broadrselectivity characteristic, an electron discharge tube provided with a cathode and a pair of anodes, 'one of the anodesjand said cathode being connected to a circuitiwhich includes. the output of said sharp' amplifierand aload resistozythe cathode and remaining anode being connected pathY connectedbetween at least onek of the tubes regulation path V,connected betweenY one of the y tubesof said broad amplifier and a point on said load resistor which is at a'positive potential with respect to said point of negative potential, a control grid'and plate associated withthe said cath- 'tron discharge tube including Within its envelope a control grid and'plate, said control grid being connected to a point of negative'potential onV said load resistor, a resistor kdisposed in the spaceV Y L ode, a resistor inthe Aspace current flowing,V

through the plate,`said control grid being connected to saidvnegative point, and a gaincontrol connection between said lastresistor and/ another tube of the broad amplifier;V ,Y

3. In a system as Vdefined in claim 1, said sharp" andV broad ainpliers each including a plurality of cascaded amplifier tubes, said eleccurrent path ,between said cathode aidgsaid plate, anda direct current connection between c a point on said last.` resistor and a gain electrode Y ofa tube i'nf said `broad' amplifier network.

accepts; Y

,Y 4. In combination'with ja sharply tuned ampli-1 fier and a broadly tuned ampliiier a pair of, diode rectifier Ycircuitshaving a common vload resistor through which flowsthe rectified signal output'V Y Y ,currents of the two amplifiers, a control tubefa connection between the cathode ofY theY control,`

tube'and a point `(Kg-negativelxrvtential on said common load resistoniayresistor inthe ,fplatef circuit of the controlY tube, again contr'ol'connection betwe'en'the cathodejof said control tube Y and the broad amplifienand' a gaincontrol con- Y nection between the plate circuit of the said control tubeand said sharp'amplifier.

V5.V Inv a radio receiver `comprising a pairl of Y parallelV amplifier networks, one ofsaid networks beingV sharply tunedand including atleast one amplier t1 1 be,'the otherfnetwork: being broadly?V i K' tuned andA including at least oneamplifier tube,

a common signal'input circuit forlsaid networks, f a diodeV rectifierY circuit connected to the output" of each network, a load resistor Vcommon to the rectier circuits and having its positiveend atV `ground potential, ajgain regulation connection from a negativeV point on said resistor Vto an amplification` control electrodeof said sharp net work tube, and a second` gain "regulation connec- `tion from aV point on said resistor, which is less Y -Y negative; than said negative point, to an amplificaton' control Velectrode of said broad network` r tube. j Y 6.V In a radio receiverjcomprising a, sharplyglv tunedraarnpliiier network in,v parallel with a 1 broadly 'tuned amplifier network, a common-signal input circuit for said network, at least one amplifier'tube ineach network, a diode rectifier'V connected to the output Yof each network, aload circuit common tothe Space current paths of said rectilers, said load circuit having its posi-V tive Yend at a relatively fixed potential and developing twoV knegative potentials `of different magnitude, means for applying the more nega-f tive of said two potentials to a gain control elec-1 trode of said sharply tuned network tube, and

Vmeans for applying the less negative of the two potentials to a gainV control electrode" ofv said broadly tuned network tube. Y Y

. RENE A. BR'ADEN.Y 

